Add a sound when you finish a task or check a checkbox
Any time something is checked, play a little sound (like a bubble popping) to add the dopamine reward effect. there could also be a setting to disable them, tough not very useful at the moment (can just mute the sound of the app)
This would increase the usability of the tasks system, and make users want to finish their tasks more. this is already used in task management apps like todoist.